Method of cooling a hydrocarbon stream and an apparatus therefor

ABSTRACT

In a method of and apparatus for, cooling a hydrocarbon stream, a hydrocarbon stream ( 45 ) to be cooled is heat exchanged in a first heat exchanger ( 50 ) against at least one refrigerant stream ( 145   b,    185   b ) having a first refrigerant stream flow rate (FR 1 ), to provide a cooled hydrocarbon stream ( 55 ) having a cooled hydrocarbon stream flow rate (FR 2 ) and at least one return refrigerant stream ( 105 ). The first refrigerant stream flow rate (FR 1 ) and the cooled hydrocarbon stream flow rate (FR 2 ) are adjusted as follows, until an inputted first set point (SP 1 ) for the first refrigerant stream flow rate (FR 1 ) is achieved. If the first set point (SP  1 ) is greater than the first refrigerant stream flow rate (FR 1 ), then the cooled hydrocarbon stream flow rate (FR 2 ) is increased before the first refrigerant stream flow rate (FR 1 ) is increased; if the first set point (SP 1 ) is less than the first refrigerant stream flow rate (FR 1 ), then the first refrigerant stream flow rate (FR 1 ) is decreased before the cooled hydrocarbon stream flow rate (FR 2 ) is decreased; and if the cooled hydrocarbon stream flow rate (FR 2 ) decreases, then the first refrigerant stream flow rate (FR 1 ) is decreased.

The present invention relates to a method of cooling a hydrocarbon stream, and an apparatus therefor.

An important example of such a hydrocarbon stream to be cooled is a natural gas stream. The cooling may include liquefying the hydrocarbon stream to produce a liquefied hydrocarbon stream, such a liquefied natural gas (LNG) stream in case when the hydrocarbon stream to be cooled is a natural gas stream.

Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a small volume and does not need to be stored at high pressure.

Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas cam then be further cooled and expanded to final atmospheric pressure suitable for storage and transportation.

In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, including but not limited to carbon dioxide, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water, other non-hydrocarbon acid gases, ethane, propane, butanes, C₅+ hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process.

Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGLs) from a natural gas stream. The methane is usually separated from the NGLs in a high pressure scrub column, and the NGLs are then subsequently fractionated in a number of dedicated distillation columns to yield valuable hydrocarbon products, either as products streams per se or for use in liquefaction, for example as a component of a refrigerant.

Meanwhile, the methane from the scrub column is subsequently liquefied to provide LNG.

US Patent Application No. 2003/0046953 discloses a method of controlling the production of LNG which permits continuous maximum utilization of the available power to drive the refrigeration cycle, because the operator can manipulate the set point of the flow rate of one of the refrigerants and the ratio of the flow rates of the heavy mixed refrigerant to the light mixed refrigerant.

The above method cannot prevent the overcooling of a heat exchanger to below its minimum temperature limits or avoid excessive mechanical stress (thermal shocks) of the heat exchanger caused when the temperature drops too quickly. Should this occur, leaks in the heat exchanger may develop. The present invention seeks to address this and other problems associated with the cooling of a hydrocarbon stream.

In a first aspect, the present invention provides a method of cooling a hydrocarbon stream in a heat exchanger, comprising at least the steps of:

(a) providing a hydrocarbon stream; (b) heat exchanging the hydrocarbon stream in a first heat exchanger against at least one refrigerant stream having a refrigerant stream flow rate, to provide a hydrocarbon stream having a hydrocarbon stream flow rate and at least one return refrigerant stream; (c) inputting a first set point for the refrigerant stream flow rate; and (d) adjusting the refrigerant stream flow rate and the hydrocarbon stream flow rate until the set point is achieved, wherein (1) if the first set point is greater than the refrigerant stream flow rate, then the hydrocarbon stream flow rate is increased before the refrigerant stream flow rate is increased; (2) if the first set point is less than the refrigerant stream flow rate, then the refrigerant stream flow rate is decreased before the hydrocarbon stream flow rate is decreased; and (3) if the hydrocarbon stream flow rate decreases, then the refrigerant stream flow rate is decreased.

In a second aspect, the present invention provides an apparatus for operating a heat exchanger, comprising at least:

a first heat exchanger having a first inlet for a hydrocarbon stream and first outlet for a cooled hydrocarbon stream, at least a second inlet for a at least one refrigerant stream and a second outlet for a return refrigerant stream;

a refrigerant flow controller to measure a signal proportional to the refrigerant stream flow rate of at least one refrigerant stream to provide a refrigerant flow signal which is transmitted to a high selector, said refrigerant flow stream controller operating a refrigerant valve to control the flow rate of the refrigerant stream;

a hydrocarbon flow controller to measure a signal proportional to the hydrocarbon stream flow rate of the hydrocarbon stream to provide a hydrocarbon flow signal which is transmitted to a low-selector, said hydrocarbon stream flow controller operating a hydrocarbon stream valve to control the flow rate of the hydrocarbon stream;

a flow setter to input a set point to provide a set point signal which is transmitted to the low selector and the high selector;

the low selector transmitting the lowest of the set point signal and hydrocarbon stream flow signal to the refrigerant flow controller; and

the high selector transmitting the highest of the set point signal and the refrigerant stream flow signal to the hydrocarbon flow controller.

Embodiments and examples of the present invention will now be described by way of example only and with reference to the accompanying non-limited drawings in which:

FIG. 1 shows schematically a flow scheme for a apparatus for cooling a hydrocarbon stream provided with means for carrying out an embodiment of the present invention;

FIG. 2 shows a control scheme for a method of cooling a hydrocarbon stream according to an embodiment of the present invention;

FIG. 3 shows a control scheme for a method of cooling a hydrocarbon stream according to a further embodiment of the present invention.

As described herein, overcooling of a heat exchanger may be prevented by adjusting the refrigerant stream flow rate and the hydrocarbon stream flow rate in the following manner, until the set point is achieved:

(1) if the first set point is greater than the refrigerant stream flow rate, then the hydrocarbon stream flow rate is increased before the refrigerant stream flow rate is increased; (2) if the first set point is less than the refrigerant stream flow rate, then the refrigerant stream flow rate is decreased before the hydrocarbon stream flow rate is decreased; and (3) if the hydrocarbon stream flow rate decreases, then the refrigerant stream flow rate is decreased.

In this way, it is ensured that there is always sufficient hydrocarbon in the hydrocarbon stream to accept the cold from the refrigerant in the refrigerant stream, thereby preventing overcooling of the heat exchanger.

It is preferred that these steps are carried out automatically i.e. without, or with minimal human intervention after the first set point is provided, for instance in a fully automated control system.

FIG. 1 provides an apparatus for cooling, preferably liquefying, a hydrocarbon stream 45. The hydrocarbon stream may be any suitable gas stream to be cooled, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually the natural gas stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 80 mol % methane.

Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. Natural gas may also contain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulphur compounds, and the like.

If necessary, the hydrocarbon stream containing the natural gas may be pre-treated before use. This pre-treatment may comprise removal of undesired components such as CO₂ and H₂S or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

In addition to methane, natural gas contains various amounts of ethane, propane and heavier hydrocarbons. The composition varies depending upon the type and location of the gas. Hydrocarbons heavier than methane generally are removed from natural gas to various extends, for several reasons, such as in case of C₅+ hydrocarbons having freezing temperatures above the liquefaction temperature of methane, that may cause them to block parts of a methane liquefaction plant. C₂₋₄ hydrocarbons can be used as a source of Liquefied Petroleum Gas (LPG).

Thus, the hydrocarbon stream 45 refers to a composition having been partly, substantially or wholly treated for the reduction and/or removal of one or more compounds or substances, including but not limited to sulphur, sulphur compounds, carbon dioxide, water, and C₂+ hydrocarbons.

If hydrocarbon stream 45 comprises natural gas, it may have been pre-treated to separate out any heavier hydrocarbons and impurities such as carbon dioxide, nitrogen, helium, water, sulphur and sulphur compounds, including but not limited to acid gases.

The hydrocarbon stream 45 may have been pre-cooled in a pre-cooling stage to reduce the temperature of the hydrocarbon stream. The provision of cooling in a pre-cooling stage is known to the person skilled in the art. The pre-cooling may be part of a liquefaction process or a separate process. Cooling of a hydrocarbon feed stream to provide the hydrocarbon stream 45 may involve reducing the temperature of the feed stream to below 0° C., for example in the range of −10° C. to −70° C. to provide a cooled initial hydrocarbon stream.

The cooled hydrocarbon feed stream can be passed to a separator such as a condensate stabilisation column, usually operating at an above ambient pressure in a manner known in the art. The condensate stabilisation column provides an overhead mixed hydrocarbon stream, preferably having a temperature below 0° C., and a heavy condensate stream. The overhead mixed hydrocarbon stream is an enriched-methane stream compared to the cooled hydrocarbon feed stream.

The term “mixed hydrocarbon stream” as used herein relates to a stream comprising methane (C₁) and at least 5 mol % of one or more hydrocarbons selected from the group comprising: ethane (C₂), propane (C₃), butanes (C₄), and C₅+ hydrocarbons. Typically, the proportion of methane in the mixed hydrocarbon stream 8 is 30-50 mol %, with significant fractions of ethane and propane, such as 5-10 mol % each.

In NGL recovery, it is desired to recover the methane in a mixed hydrocarbon stream for further cooling, such as liquefaction in a LNG plant, and to provide at least a C2+ stream, optionally one or more of a C₂ stream, a C₃ stream, a C₄ stream, and a C₅+ stream.

At least a fraction, usually all, of the mixed hydrocarbon stream is passed into a NGL recovery system. The NGL recovery system usually involves one or more gas/liquid separators such as distillation columns to separate the mixed hydrocarbon stream into at least a C₁ stream and one or more C₂+ streams, commonly at low pressure, for example in the range of 20 to 35 bar. An example of a suitable first gas/liquid separator is a “demethanizer” designed to provide a methane-enriched overhead stream, and one or more liquid streams at or near the bottom enriched in C₂+ hydrocarbons.

As the mixed hydrocarbon stream 8 is usually provided from a high pressure 40 to 70 bar initial hydrocarbon stream, it may require to be expanded, for instance to reduce the temperature, prior to the first gas/liquid separator.

The first gas/liquid separator is adapted to separate the liquid and vapour phases, so as to provide a C₁ overhead stream (as the hydrocarbon stream 45 subsequently used herein), and a C₂+ bottom stream. The C1 overhead stream (which is the hydrocarbon stream 45) may still comprise a minor (<10 mol %) amount of C₂+ hydrocarbons, but is preferably >80 mol %, more preferably >95 mol % methane. The C₂+ bottom stream 50 can be >90 or >95 mol % ethane and heavier hydrocarbons, and can be subsequently fractionated or otherwise used in a manner known in the art for an NGL stream.

A scheme for cooling, preferably liquefying, a hydrocarbon stream such as natural gas is shown in FIG. 1. The hydrocarbon stream 45 is passed through a main cooling stage 1 having first heat exchanger 50, to provide a cooled, preferably liquefied hydrocarbon stream 55, which can be liquefied natural gas.

The main cooling stage 1 comprises at least one, preferably cryogenic, first heat exchanger 50. The first heat exchanger 50 may be a plate and fin or shell and tube heat exchanger, more preferably a kettle heat exchanger. The first heat exchanger 50 has a shell side 51. In the shell side can be arranged three tube bundles 53, 57, 59. The main cooling stage 1 further comprises a refrigerant circuit 100 comprising a refrigerant compressor 110, a suitable refrigerant driver 120, a refrigerant cooler 130 and a separator 140.

There can be various arrangements for the hydrocarbon stream 45 and the refrigerant stream in the main cooling stage 1. Such arrangements are known in the art. These can involve one or more heat exchangers 50, optionally at different pressure levels, and optionally within one vessel such as the cryogenic heat exchanger shown.

In the embodiment shown in FIG. 1, hydrocarbon stream 45 is passed through the first heat exchanger 50 in first tube bundle 52. The first heat exchanger 50 reduces the temperature of the hydrocarbon stream 45 to provide a cooled, preferably liquefied hydrocarbon stream 55 such as a LNG stream, which could be at a temperature of about or lower than −90° C., preferably lower than −120° C.

The liquefied hydrocarbon stream 55 can be passed to an expansion device, such as a cooled hydrocarbon stream valve 60 which is a flow control valve, optionally preceded by an expansion turbine (not shown), to control the flow rate of the cooled hydrocarbon stream 55. The cooled hydrocarbon stream valve 60 can lower the pressure of the cooled hydrocarbon stream 55, for instance to allow the storing of a LNG stream at about atmospheric pressure.

At least one refrigerant stream 145 b, 185 b is used to remove heat from the hydrocarbon stream 45 in the first heat exchanger 50. A refrigerant, preferably being a mixed refrigerant, is cycled in the refrigerant circuit 100. FIG. 1 shows a closed refrigerant cycle.

In one embodiment of the present invention, the mixed refrigerant for the refrigerant circuit 100 comprises:

>30 mol % of a compound selected from the group consisting of ethane and ethylene or a mixture thereof; and

>30 mol % of a compound selected from the group consisting of propane and propylene or a mixture thereof. In general, the second refrigerant may be any suitable mixture of components including two or more of nitrogen, methane, ethane, ethylene, propane, propylene, butane, pentane, etc.

A gaseous refrigerant is drawn from the shell side 51 of first heat exchanger 50 as return refrigerant stream 105. This is compressed by refrigerant compressor 110 to provide a compressed refrigerant stream 115. The refrigerant compressor 110 is driven by a suitable driver refrigerant compressor driver 120.

Compressed refrigerant stream 115 is passed to a cooler 130, such as an air cooler, in which the heat of compression is removed together with heat absorbed in the first heat exchanger 50, and the mixed refrigerant can thus be partially condensed providing a cooled compressed refrigerant stream 135. The cooling and partial condensation of the compressed refrigerant stream 115 may also be carried out in one of more heat exchangers.

The cooled compressed refrigerant stream 135 is passed to a separator 140, which splits the cooled compressed refrigerant stream 135 into one or more fractions. FIG. 1 shows the cooled compressed refrigerant stream 135 split into two fractions, an incoming refrigerant stream 143 and a second incoming refrigerant stream 147. Preferably, separator 140 splits the cooled compressed refrigerant stream 135 into bottoms heavy mixed refrigerant (HMR) 143 and an overhead light mixed refrigerant (LMR) 147. If separator 140 is a gas/liquid separator, the HMR fraction can be a liquid product and the LMR fraction can be a vapour product.

The incoming refrigerant stream 143, which can be a first or heavy mixed refrigerant, is passed through the first heat exchanger 50 in second tube bundle 57, in which it can be sub-cooled. The incoming second refrigerant stream 147, which can be a second or light mixed refrigerant, is passed through the first heat exchanger 50 in third tube bundle 59, in which it can be liquefied and sub-cooled.

The first or heavy mixed refrigerant exits the second tube bundle 57 as cooled refrigerant stream 145. Cooled refrigerant stream 145 is expanded in a refrigerant expander 150 to provide an expanded refrigerant stream 145 a. Refrigerant expander 150 can be driven by a suitable refrigerant expander driver 160. The expanded refrigerant stream 145 a can be passed through a refrigerant valve 170, which can control the flow rate of the expanded refrigerant stream 145 a, to provide refrigerant stream 145 b, which is a controlled stream. The refrigerant stream 145 b is passed to the shell side 51 of the first heat exchanger 50 via second inlet 176 to cool hydrocarbon stream 45 in first tube bundle 53.

Similarly, the second or light mixed refrigerant exits the third tube bundle 59 as cooled second refrigerant stream 185. Cooled second refrigerant stream 185 is expanded in a second refrigerant expander 190 to provide an expanded second refrigerant stream 185 a. Second refrigerant expander 190 can be driven by a suitable second refrigerant expander driver 200. The expanded second refrigerant stream 185 a can be passed through a second refrigerant valve 210, which can control the flow rate of the expanded cooled second refrigerant stream 185 a, to provide a second refrigerant stream 185 b, which is a controlled stream. The second refrigerant stream 185 b is passed to the shell side 51 of the first heat exchanger 50 via third inlet 216 to cool hydrocarbon stream 45 in first tube bundle 53.

The method of cooling the hydrocarbon stream 45 is controlled in the following way:

A refrigerant stream flow rate, FR1, corresponding to the flow rate of the refrigerant stream 145 b is measured by a refrigerant flow controller FC1 (340). FIG. 1 shows the refrigerant flow controller 340 placed to measure the flow rate FR1 of the cooled refrigerant stream 145. However, the refrigerant flow controller 340 could be situated to measure the flow rate of any of the streams 143, 145, 145 a, 145 b, as long as the flow controller 340 provides a signal proportional to the flow rate FR1 of the refrigerant which is passed into the shell side 51 of the first heat exchanger 50 to cool hydrocarbon stream 45 e.g. the flow rate FR1 of refrigerant stream 145 b which is passed to the shell side 51 of the first heat exchanger 50 via second inlet 176 to cool the hydrocarbon stream 45.

A hydrocarbon stream flow rate, FR2, corresponding to the flow rate of the cooled hydrocarbon stream 55 is measured by a hydrocarbon flow controller FC2 (350). FIG. 1 shows the cooled hydrocarbon flow controller 350 placed to measure the flow rate FR2 of the cooled hydrocarbon stream 55. However, the hydrocarbon flow controller 350 could be situated to measure the flow rate FR2 of hydrocarbon stream 45, or any other hydrocarbon stream as long as the flow controller 350 provides a signal proportional to the flow rate FR2 of the hydrocarbon stream passing through the first heat exchanger 50.

The measurement of stream flow can be carried out by any suitable apparatus, unit or device known in the art. Non-limiting examples include orifice plates, venturi tubes, flow nozzles, variable area meters, pilot tubes, calorimetric meters, turbine meters, coriolis meters, ultrasonic Doppler meters and vortex meters.

The flow controllers also control the operation of a means for controlling the flow of a stream, preferably a valve, such as a pneumatically, hydraulically or electrically actuated valve.

A first set point, SP1, for the refrigerant stream flow rate FR1 is selected and input to a flow setter HC (300). The first set point, SP1, although provided in terms of the refrigerant stream flow rate FR1 corresponds to the desired output of cooled hydrocarbon stream 55, which is preferably a LNG stream, from the first heat exchanger 50.

Overcooling of the first heat exchanger 50 can occur when more cooling duty from the refrigerant is supplied than is required by the hydrocarbon to be cooled. In order to avoid the overcooling of the first heat exchanger 50, the refrigerant stream flow rate FR1 and the hydrocarbon stream flow rate FR2 are adjusted according to the following cross-limiting control method.

In the event that the first set point SP1 for the refrigerant stream flow rate is greater than the measured refrigerant stream flow rate FR1 i.e. when the cooled hydrocarbon output of the first heat exchanger 50 is to be increased, then the hydrocarbon stream flow rate FR2 is increased before the refrigerant stream flow rate FR1 is increased.

As long as the increase in the flow rate of the refrigerant stream FR1 follows the increase in the flow rate of the hydrocarbon stream FR2 overcooling of the first heat exchanger 50 can be avoided. However, any period of time between increasing the flow rate of the hydrocarbon stream FR2 and increasing the flow rate of the refrigerant stream FR1 should be as short as possible in view of the system response time in order to prevent undercooling of the hydrocarbon stream.

In the event that the first set point SP1 for the refrigerant stream flow rate is less than the measured refrigerant stream flow rate FR1 i.e. when the cooled hydrocarbon output of the first heat exchanger 50 is to be decreased, then the refrigerant flow rate FR1 is decreased before the hydrocarbon stream flow rate FR2 is decreased.

As long as the decrease in the flow rate of the hydrocarbon stream FR2 follows the decrease in the flow rate of the refrigerant stream FR1, overcooling of the first heat exchanger 50 can be avoided. However, any period of time between decreasing the flow rate of the refrigerant stream FR1 and decreasing the flow rate of the hydrocarbon stream FR2 should be as short as possible in view of system response time in order to avoid undercooling of the hydrocarbon stream.

In the event that the hydrocarbon stream flow rate FR2 decreases, for instance during a tripping event in which a user of the cooled hydrocarbon stream 55 is withdrawn from operation or a supplier of the hydrocarbon stream 45 is withdrawn from operation, then the refrigerant stream flow rate FR1 should also be decreased.

Preferably the refrigerant stream flow rate FR1 is adjusted proportionally with the hydrocarbon stream flow rate FR2, such that a constant temperature for the cooled hydrocarbon stream 55 is maintained.

The refrigerant flow controller 340 can control the flow rate of the refrigerant stream FR1 by operating refrigerant stream valve 170. In an alternative embodiment (not shown), the flow rate of the refrigerant stream FR1 could be controlled by the presence of a control valve in any of the streams 143 or 145 as long as the control valve effected the flow of refrigerant into the shell side 51 of the first heat exchanger 50.

Similarly the cooled hydrocarbon flow controller FC2 can control the flow rate of the cooled hydrocarbon stream 55 (and thereby the flow rate of the hydrocarbon stream 45) by operating cooled hydrocarbon valve 60. In an alternate embodiment (not shown), the flow rate of the cooled hydrocarbon stream 55 could be controlled by the presence of a control valve in the hydrocarbon stream 45.

It will be apparent from the system of FIG. 1 that the flow rate of any second refrigerant stream FR3 must also be controlled. FIG. 1 shows a second refrigerant flow controller FC3 (360) which operates a second refrigerant flow valve 210 in order to control the flow rate FR3 of the second refrigerant stream 185 b. The second refrigerant flow valve 210 is shown after the second refrigerant expander 190, but can be placed in another second refrigerant stream, such as 147 or 185 as long as the flow rate of second refrigerant into the shell side 51 of the first heat exchanger 50 can be controlled.

The flow rate FR3 of the second refrigerant stream 185 b is adjusted relative to, and proportionally with, the flow rate of the refrigerant stream FR1. A refrigerant controller 330 can be used to adjust the position of the second refrigerant flow valve 210 based upon the instructions provided to the first refrigerant stream flow controller FC1. The second refrigerant flow controller FC3 measures the second refrigerant flow rate FR3 in order to ensure that the system is providing the correct cooling duty to the first heat exchanger 50.

The cross-limiting control system which can be used in the method described herein can be automatic. FIG. 1 shows Flow controllers FC1 (340), FC2 (350) and FC3 (360) in operation of flow control valves 170, 60, 210 respectively. The first set point SP1 for the flow rate of the first refrigerant stream flow rate (FR1) is input to a flow setter HC (300). The flow setter 300 receives the input of the first set point SP1 and transmits this to low selector 310 and high selector 320.

The low selector 310 receives the cooled hydrocarbon stream flow rate as a signal from the cooled hydrocarbon stream flow controller 350. The high selector 320 receives the refrigerant stream flow rate as a signal from the refrigerant stream flow controller FC1.

The nature and operation of the high and low selectors 310, 320 will now be discussed in greater detail with respect to FIG. 2. FIG. 2 shows a representation of a control scheme for a method of cooling a hydrocarbon as described herein. A main cooling stage 1 as described for FIG. 1 can be used with this embodiment. Only cooled hydrocarbon stream 55 and corresponding control valve 60, and cooled refrigerant stream 45, refrigerant compressor 150, compressed refrigerant stream 145 a, control valve 170 and refrigerant stream 145 b from FIG. 1 are shown in FIG. 2 for simplicity. However, the additional features of FIG. 1 may also be present.

The first set point SP1 for the refrigerant stream flow rate FR1 is input into a flow setter HC (300) which generates a set point signal SPS, which is transmitted to the low selector 310 and the high selector 320.

Refrigerant flow controller 340 generates a refrigerant flow signal FS1 which is proportional to the flow rate FR1 of the refrigerant stream 145 b. The refrigerant flow signal FS1 is transmitted to a high selector 320. High selector 320 also receives a first set point signal SPS from the flow setter 300.

The refrigerant flow controller 340 operates the refrigerant stream valve 170 to control the flow rate of the refrigerant stream 145 b.

Hydrocarbon flow controller 350 generates a hydrocarbon flow signal FS2 which is proportional to the flow rate FR2 of the cooled hydrocarbon stream 55. The hydrocarbon flow signal FS2 is transmitted to the low selector 310. The low selector 310 also receives the first set point signal SPS from the flow setter 300.

The cooled hydrocarbon flow controller 350 operates a cooled hydrocarbon stream valve 60 to control the flow rate of the cooled hydrocarbon stream 55.

The low selector 310 is programmed to pass the lowest of the set point signal SPS and hydrocarbon flow signal FS2 to the first refrigerant flow controller 340. In this way, an increase in the first set point SPS will only lead to an increase in the flow rate of the refrigerant stream FR1 after the flow rate FR2 of the hydrocarbon stream has been increased.

The high selector 320 is programmed to pass the highest of the set point signal SPS and the refrigerant flow signal FS1 to the hydrocarbon flow controller 350. In this way, a decrease in the set point SPS will only lead to a decrease in the flow rate FR2 of the hydrocarbon stream after the flow rate FR1 of the refrigerant stream has been decreased.

Thus, a method of cooling a hydrocarbon stream is provided in which there is cross-limiting control between the flow rate FR2 of the hydrocarbon stream and the flow rate FR1 of the refrigerant stream such that overcooling of the first heat exchanger 50 is prevented.

FIG. 3 shows a representation of a control scheme for a method of cooling a hydrocarbon as described herein in which the temperature TC2 of the cooled hydrocarbon stream 55 can be maintained by adjusting the ratio of the flow rate FR1 of the refrigerant stream 145 b, such as a heavy mixed refrigerant, compared to the flow rate FR2 of the cooled hydrocarbon stream 55. It will be apparent that in the embodiments of FIG. 1 and FIG. 2, there was no means for adjusting this ratio and it was therefore fixed.

The cooled hydrocarbon stream 55 is provided with a temperature controller, TC2 (370). The temperature controller 370 measures the temperature of the cooled hydrocarbon stream 55 and transmits a signal TS2 proportional to the temperature.

The temperature of the cooled hydrocarbon stream TC2 can be adjusted by providing a temperature set point TSP to the temperature controller 370. The temperature TC2 of the cooled hydrocarbon stream 55 can be decreased by decreasing the flow rate FR2 of the cooled hydrocarbon stream 55 relative to a refrigerant stream flow rate FR1. Similarly, the temperature TC2 of the cooled hydrocarbon stream 55 can be increased by increasing the flow rate FR2 of the cooled hydrocarbon stream 55 relative to a constant refrigerant stream flow rate FR1.

The signal TS2 from the temperature controller 370 can modulate the signal from the high selector 320 to the cooled hydrocarbon stream controller FC2 to either increase or reduce the flow rate of the cooled hydrocarbon stream 55 compared to an unmodulated signal. However, the modulation carried out to the signal from the cooled hydrocarbon stream flow controller FC2 to the low selector 310 is the inverse of the modulation carried out on the signal from the high selector 320, such that the cooled hydrocarbon stream flow signal FS2 reaching the low selector 310 corresponds to the signal from the hydrocarbon flow controller FC2 which would have occurred if it was unmodulated by the cooled hydrocarbon stream temperature controller TC2. In this way, the operation of the low selector 310, and therefore the refrigerant flow controller FC1 is unaffected by the cooled hydrocarbon stream temperature controller TC2.

FIG. 3 shows one way in which the signal TS2 from the temperature controller 370 can modulate the signal from the High selector 320. The system has a given ratio of the flow rate of the cooled hydrocarbon stream (LNG) to the refrigerant stream (HMR), which provides the cooled hydrocarbon stream 55 at a specific temperature. This ratio is shown as (LNG/HMR) in FIG. 3. In order to be able to adjust the temperature of the cooled hydrocarbon stream 55, the ratio of the flow rate of the cooled hydrocarbon stream 55 to the refrigerant stream must be altered from the given ratio. A parameter b derived from the signal TS2 from the temperature controller 370 can be used to scale the signal from the high selector 320.

For instance, a parameter c derived from the signal provided to the hydrocarbon flow controller 350 can be determined as a function of a parameter a derived from the signal from the high selector 320, the ratio of the cooled hydrocarbon flow rate LNG to refrigerant flow rate HMR i.e. (LNG/HMR) and a scaling factor (b/100) determined from the parameter b derived from the signal from the temperature controller 370. When parameter b exceeds 100, for example when parameter b is in the range of >100 to 150, parameter c of the signal provided to the hydrocarbon flow controller 350 will increase accordingly.

Similarly, a parameter e derived from the signal provided to the Low selector 310 can be determined as a function of the parameter d derived from the signal from the hydrocarbon flow controller 350, the ratio of the refrigerant flow rate HMR to the cooled hydrocarbon flow rate LNG i.e. (HMR/LNG) and the inverse of the scaling factor (b/100) i.e. (100/b) determined from the parameter b derived from the signal from the temperature controller 370.

The flow rate of any second refrigerant stream FR3 can be varied in proportion with that of the flow rate FR1 of the refrigerant stream to maintain the ratio of refrigerant to second refrigerant such as the ratio of HMR to LMR. In a further embodiment, FIG. 3 shows a refrigerant bypass stream 225, which bypasses refrigerant expander 150. The refrigerant bypass stream 225 is controlled by refrigerant bypass valve 230 to provide controlled refrigerant bypass stream 225 a. Controlled refrigerant bypass stream 225 a can be combined with refrigerant stream 145 b to provide combined refrigerant stream 245. The refrigerant bypass valve 230 can be operated by a signal from the refrigerant flow controller 340 to enable the cooled refrigerant stream 145 to bypass the refrigerant expander 150.

A person skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims. 

1. A method of cooling a hydrocarbon stream in a heat exchanger, comprising at least the steps of: (a) providing a hydrocarbon stream; (b) heat exchanging the hydrocarbon stream in a first heat exchanger against at least one refrigerant stream having a refrigerant stream flow rate, to provide a cooled hydrocarbon stream having a hydrocarbon stream flow rate and at least one return refrigerant stream; (c) inputting a first set point for the refrigerant stream flow rate; and (d) adjusting the refrigerant stream flow rate and the hydrocarbon stream flow rate until the set point is achieved, wherein: (d1) if the first set point is greater than the refrigerant stream flow rate, then the hydrocarbon stream flow rate is increased before the refrigerant stream flow rate is increased; (d2) if the first set point is less than the refrigerant stream flow rate, then the refrigerant stream flow rate is decreased before the hydrocarbon stream flow rate is decreased; and (d3) if the hydrocarbon stream flow rate decreases, then the refrigerant stream flow rate is decreased.
 2. The method according to claim 1, wherein the adjusting of the refrigerant stream flow rate and the hydrocarbon stream flow rate in step (d) is automatic.
 3. The method according to claim 1, wherein the ratio: [refrigerant stream flow rate]/[hydrocarbon stream flow rate] is maintained at or below a preselected level during step (d).
 4. The method according to claim 1, wherein: the refrigerant stream flow rate is measured by a refrigerant flow controller which generates a refrigerant flow signal which is transmitted to a high selector, said refrigerant flow controller operating a refrigerant stream valve to control the flow rate of the refrigerant stream; the hydrocarbon stream flow rate is measured by a hydrocarbon flow controller which generates a hydrocarbon flow signal which is transmitted to a low-selector, said hydrocarbon flow controller operating a hydrocarbon stream valve to control the flow rate of a hydrocarbon stream; the first set point is input into a flow setter which generates a set point signal, which is transmitted to the low selector and the high selector; the low selector passes the lowest of the set point signal and hydrocarbon flow signal to the refrigerant flow controller; and the high selector passes the highest of the set point signal and the refrigerant flow signal to the cooled hydrocarbon flow controller.
 5. The method according to claim 1, wherein the at least one refrigerant stream is selected from the group comprising a heavy mixed refrigerant stream and a light mixed refrigerant stream.
 6. The method according to claim 1, further comprising, in step (b), heat exchanging the hydrocarbon stream against a second refrigerant stream having a second refrigerant stream flow rate in the first heat exchanger.
 7. The method according to claim 6, wherein the second refrigerant stream flow rate is determined as a proportion of the refrigerant stream flow rate.
 8. The method according to claim 6, further comprising: a second refrigerant flow controller which operates the second refrigerant stream valve thereby changing the flow rate of the second refrigerant stream; and a refrigerant controller which receives the lowest of the set point signal and cooled hydrocarbon flow signal from the low selector and adjusts the flow rate of the second refrigerant stream relative to the flow rate of the refrigerant stream by transmitting a refrigerant controller signal to the second refrigerant flow controller.
 9. The method according to claim 6, wherein the second refrigerant stream is a light mixed refrigerant stream when the refrigerant stream is a heavy mixed refrigerant stream or the second refrigerant stream is a heavy mixed refrigerant stream when the refrigerant stream is a light mixed refrigerant stream.
 10. The method according to claim 1, further comprising the steps of: (i) cooling an incoming refrigerant stream in the first heat exchanger to provide a cooled refrigerant stream; (ii) expanding the cooled refrigerant stream in a refrigerant expander to provide an expanded refrigerant stream; (iii) passing the expanded refrigerant stream through a refrigerant valve to provide the refrigerant stream; and (iv) passing the refrigerant stream to a second inlet of the first heat exchanger.
 11. The method according to claim 10, further comprising the steps of: (v) passing a return refrigerant stream to a refrigerant compressor to provide a compressed refrigerant stream; (vi) cooling the compressed refrigerant stream in a cooler to provide a cooled compressed refrigerant stream; and (vii) separating the cooled compressed refrigerant stream in a separator to provide at least one incoming refrigerant stream.
 12. The method according to claim 11, wherein separating the cooled compressed refrigerant stream in step (vii) additionally produces a second incoming refrigerant stream, the method comprising the further steps of: (viii) cooling the second incoming refrigerant stream in the first heat exchanger to provide a cooled second refrigerant stream; (ix) expanding the cooled second refrigerant stream in a second refrigerant expander to provide an expanded second refrigerant stream; (x) passing the expanded second refrigerant stream through a second refrigerant valve to provide a second refrigerant stream; and (xi) passing the second refrigerant stream to a third inlet of the first heat exchanger.
 13. The method according to claim 1, wherein the hydrocarbon stream is a natural gas stream and the cooled hydrocarbon stream is a LNG stream.
 14. Apparatus for operating a heat exchanger, comprising at least: a first heat exchanger having a first inlet for a hydrocarbon stream and first outlet for a cooled hydrocarbon stream, at least a second inlet for a at least one refrigerant stream and a second outlet for a return refrigerant stream; a refrigerant flow controller to measure a signal proportional to the refrigerant stream flow rate of at least one refrigerant stream to provide a refrigerant flow signal which is transmitted to a high selector, said refrigerant flow stream controller operating a refrigerant valve to control the flow rate of the refrigerant stream; a cooled hydrocarbon flow controller to measure a signal proportional to the cooled hydrocarbon stream flow rate of the cooled hydrocarbon stream to provide a cooled hydrocarbon flow signal which is transmitted to a low-selector, said cooled hydrocarbon stream flow controller operating a cooled hydrocarbon stream valve to control the flow rate of the cooled hydrocarbon stream; a flow setter to input a set point to provide a set point signal which is transmitted to the low selector and the high selector; the low selector transmitting the lowest of the set point signal and cooled hydrocarbon stream flow signal to the refrigerant flow controller; and the high selector transmitting the highest of the set point signal and the refrigerant stream flow signal to the cooled hydrocarbon flow controller.
 15. The apparatus according to claim 14, further comprising: a third inlet for a second refrigerant stream in the first heat exchanger; a second refrigerant flow controller which operates a second refrigerant stream valve to change the flow rate of the second refrigerant stream; and a refrigerant controller which receives the lowest of the set point signal and cooled hydrocarbon flow signal from the low selector and adjusts the flow rate of the second refrigerant stream relative to the flow rate of the refrigerant stream by transmitting a refrigerant controller signal to the second refrigerant flow controller.
 16. The method according to claim 1, wherein the at least one return refrigerant stream is drawn from the first heat exchanger as a gaseous refrigerant, further comprising: compressing the gaseous refrigerant to provide a compressed refrigerant stream; partially condensing the compressed refrigerant stream thereby providing a cooled compressed refrigerant stream; passing the cooled compressed refrigerant stream to gas/liquid separator; splitting the cooled compressed refrigerant stream into a heavy mixed refrigerant stream in liquid form and an overhead light mixed refrigerant stream in vapour form, wherein the at least one refrigerant stream is said heavy mixed refrigerant stream. 